Oil diffusion vacuum pump

ABSTRACT

An oil diffusion pump which has a plurality of coaxially arranged cylindrical chimneys extending from the bottom of a cylindrical envelope. The bottom of said envelope has hollow projections extending upwardly between the chimneys and a heating element has portions thereof extending into said hollow projections.

United States Patent [191 Abe Jan. 1, 1974 OIL DIFFUSION VACUUM PUMP OTHER PUBLICATIONS T A H [75] Inventor am) me]! Japan H. A. Stemherz, High-Vacuum Technology and [73] Assignee: Ayumi Industries Company, Equipment, reprinted from Chemical Engineering,

Limited, Kobe, Japan Aug. 20, 1962. [22] Filed: Nov. 13, 1972 Primary ExaminerAl Lawrence Smith [21 1 Appl' 305938 Assistant Examiner-Richard E. Gluck Attorney-Eugene E. Geoffrey, Jr. [30] Foreign Application Priority Data May 10, 1972 Japan 47-46698 [57] ABSTR CT U-S- Cl. An diffusion pump has a of ceaxi- [5 arranged cylindrical chimneys; extending from the [58] F wk! of Search 417/152, 153, 154; bottom f a cylindrical enve]ope The bottom f Said 219/530 540; 122/13 A envelope has hollow projections extending upwardly between the chimneys and a heating element has porl References Cited tions thereof extending into said hollow projections.

FOREIGN PATENTS OR APPLICATIONS 1 Claim, 8 Drawing Figures 12/1937 Great Britain ..417/l53 4 //A H o I0 of iiiij ijiiiiliiii ii 2 \7-\-.

5 I MMMQ'MQ PATENTED 1 74 SHEEF 2 N3 FIG. 3

OIL DIFFUSION VACUUM PUMP This invention relates to an improved oil diffusion vacuum pump and more particularly to a diffusion pump of the fractional distillation type.

A conventional oil diffusion pump of this type comprises a cylindrical envelope having a flat bottom and a plurality of cylindrical distillation towers standing erect and coaxially in the envelope. The upper portion of the casing is referred to as a condenser and is provided with a cooling water pipe wound thereon and the lower portion thereof is referred to as a boiler and is provided with an electric heating element attached to the bottom. The boiler contains an appropriate quantity of diffusion pump oil having a lower vapor pressure.

It is well known that in the diffusion pump of this type it is essential to maintain the oil surface as steady as possible in order to maintain the best condition for the exhaust characteristic of the pump. On the other hand, however, it is also essential to heat the oil as high as possible in order to accelerate diffusion of the oil to increase the exhaust speed. Thus, the diffusion oil in the boiler is kept at a temperature just below the boiling point. Therefore, prior diffusion pumps of this type have had the disadvantage that fluctuations of the power source for the heating element disturb the oil surface and sometimes causes bumping of the oil which remarkably deteriorates the exhaust speed, ultimate pressure and other pumping characteristics of the pump. While such a disadvantage is caused by a large temperature difference of the oil between the surface and bottom, this temperature difference also causes undesirable decomposition of the diffusion oil which not only adversely affects the pumping characteristics but also materially reduces the life of the oil.

Therefore, an object of this invention is to provide an improved oil diffusion pump of fractional distillation type which overcomes the foregoing disadvantages by providing its boiler with a novel and improved structure which can substantially avoid a temperature difference in the oil as mentioned above.

According to this invention, the bottom of the boiler has a plurality of annular upwardly extending hollow projections arranged concentrically so as to be located respectively between the respective distillation towers, and the heating element of the boiler has a plurality of annular projections arranged concentrically to fit in the annular grooves of the hollow projections of the boiler.

Other objects and features of this invention will be described further in detail hereinunder with reference to the accompanying drawings.

In the Drawings:

FIG. 1 is a schematic cross-sectional view of a prior art oil diffusion pump of fractional distillation type;

FIG. 2 is a schematic cross-sectional side view of an embodiment of an oil diffusion pump according to this invention;

FIG. 3 is a fragmentary cross-sectional side view of the boiler of the diffusion pump of FIG. 2;

FIG. 4 is a bottom view of the boiler of FIG. 3;

FIG. 5 is a perspective view of an embodiment of the heating element for use with the boiler of FIGS. 3 and 4; and

FIGS. 6, 7 and 8 are diagrams illustrating the test results and the improved characteristics of the diffusion pump of this invention.

Throughout the drawings like reference numerals are used to denote corresponding structural components.

Referring now to FIG. 1, the prior art diffusion pump includes a cylindrical envelope 1 which may be made of glass or metal, the lower portion 2 of which the so called boiler has a flat bottom 3 which is preferably made of metal and hermetically sealed to the envelope 1. The upper end of the envelope 1 is provided with a hermetically sealed metal flange 4 for coupling the pump to a chamber and constitutes an inlet port 7. The cylindrical envelope 1 has a branch pipe 5 having at hermetically sealed metal flange 6 at the end which is to be coupled to a fore-pump, such as mechanical rotary vacuum pump, to constitute an output port 8.

In the envelope 1, three coaxial upright metal cylinders 9, l0 and I] referred to as chimneys have heights which are highest in the center and successively decrease toward the outside. The chimneys 9, l0 and 11 have umbrellas 12, 13 and 14 respectively supported above them to form nozzles 15, 16 and 17 respectively therebetween. At the foot of the chimneys 9, 10 and 11, holes 18, 19 and 20 are formed in diametrical alignment and on alternate sides. An appropriate quantity of diffusion pump oil 21 is contained in the boiler 2.

Under the bottom 3 of the boiler 2, a heating element 22 is positioned in intimate contact therewith. The heating element 22 is made of a material having high heat conductivity, such as metal, and includes an electric heater 23 which is electrically insulated and embedded in the material. A helical cooling water pipe 24 is wound on the upper portion of envelope 1 and is hereinafter referred to as a condenser.

In operation, the oil 21 is heated by the heating element 22 to evaporate it and the vapor flows upwardly in the distillation towers or chimneys 9, 10 and I1 and then outwardly and downwardly through nozzles I5, 16 and 17. Therefore, the gas in the envelope 1 moves downwardly with the oil vapor and is exhausted from the outlet 8 but the oil vapor cooled and condensed by the cooling pipe 24 falls into the boiler 21. During this circulation the oil 21 moves from the outside into the center passing through the holes 20, 19 and 18 and around the feet of the chimneys ll, 10 and 9 successively.

In such a prior art diffusion pump, the diffusion pump oil 21 is heated only from the bottom and there is a substantial temperature difference between the surface and bottom which badly affects the various characteristics of the pump as described above.

Referring to FIG. 2 representing an embodiment of this invention, the bottom 3 of the boiler 2 is provided with upward hollow projections 25, 26 and 27 which are recessed from the underside. While the projection 25 is cylindrical and stands in the center of the chimney 9, the other projections 26 and 27 each have a circular' shape with a narrow portion omitted to form a gap 31 as shown in FIGS. 3 and 4 and are positioned between the chimneys 9 and 10 and 10 and 11 respectively. The diffusion pump oil 21 is at a level approximately midway of the height of the projections. In this case the holes 18, 19 and 20 of the chimneys 9, 10 and 11 are formed in the same side along a radius extending in a direction opposite to the gaps 31 of the projections 26 and 27 so that the oil 21 flows excursively between the chimneys and the projections from the outside to the center as readily understood.

The recesses of the projections 25, 26 and 27 receive projections 28, 29 and 30 respectively which are formed integrally with the heating element 22 and shaped similarly to the projections 25, 26 and 27 so that intimate contact is established between the inner walls of the underside recesses of the projections 25, 26 and 27 and the surfaces of the projections 28, 29 and 30 as shown in further detail in FIG. 5. In FIG. the gap 34 corresponds to the gap 31 in FIG. 4 and numerals 32 and 33 represent terminals of the heater 23 which is insulated from and embedded in the body of the heating element 22. The projections 28, 29 and 30 of the heating element 22 are preferably made of the same material as the body of the element which has a high heat conductivity.

With the novel structure of the boiler according to this invention, the temperature difference between the surface and bottom of the oil 21 is very small and disturbance and bumping of the oil disappears completely. Moreover, decomposition of the oil due to excessive heating is prevented. Thus, the exhausting ability of the diffusion pump has been remarkably improved as will be described.

In the graphic charts of FIGS. 6, 7 and 8, percent deviation of the electric power supplied to the heater 23 from the optimum value is indicated along the abscissa and various exhausting characteristics are indicated along the ordinates. Throughout these charts, Curve A having clear circles was obtained from a diffusion pump having a boiler according to this invention and Curve B having dark circles was obtained from a similar diffusion pump having a boiler with flat bottom according to the prior art. Curve C having dark squares in FIGS. 6 and 8 was obtained from a boiler which has projections according to this invention but having no recess on the underside and, therefore, the heating element is the same as that of prior art which does not have projections.

FIG. 6 shows the variation of the exhaust speed in percentage with reference to the highest speed at the optimum condition. As shown, this invention has increased the pumping speed of the diffusion pump by as much as 30 percent and also reduced fluctuation of the exhaust speed due to power fluctuations.

FIG. 7 shows the variation of the pressure in percentage with reference to the lowest pressure at each power which was measured as 2 to 3 X Torr for this invention and 4 to 5 X 10 Torr for the prior artv As shown, the pressure variation has been reduced to not less than one percent at the optimum power condition and, moreover, reduced further under a fluctuating power condition.

FIG. 8 shows the ultimate pressure which has beem improved remarkably by this invention.

Further, the back streaming of diffusion oil was monitored for three hours and the measured value was 9 X 10 milligrams per minute per square centimeter for this invention as compared to 1.5 X 10' millimeter per minute per square centimeter for the prior art.

As described above, the disadvantages such as decomposition, bumping and back streaming in the prior art device could be completely removed and a very low and stable ultimate pressure could be obtained by this invention. Moreover, as the heating element of this invention has a large heating surface area, the warmingup time has been shortened and the necessary power consumption has been reduced to one third.

While the test of measurement for this invention was carried out on a heating element including a heater embedded only in the body of the element but not in the projections, further improved characteristics can be expected on a heating element having a heater embedded also in the projections, which is also included in the scope of this invention.

What is claimed is:

1. An oil diffusion vacuum pump comprising a cylindrical envelope having a flat bottom, a plurality of chimneys coaxially arranged in said envelope, nozzle means formed on top of each of said chimneys, inlet means in said envelope upstream of said nozzles, outlet means in said envelope downstream of said nozzles and a heating element attached to the bottom of said envelope, wherein said bottom of the envelope is provided with a plurality of concentric annular hollow projections which are recessed from the underside and arranged so that said projections are located between said chimneys, and said heating element is provided with a plurality of concentric annular projections which are formed integrally with the body of said element and shaped to fit within the recesses of said hollow projections on the bottom of said envelope, said projections of the heating element being positioned in said recesses and in thermally coupled relationship thereto. 

1. An oil diffusion vacuum pump comprising a cylindrical envelope having a flat bottom, a plurality of chimneys coaxially arranged in said envelope, nozzle means formed on top of each of said chimneys, inlet means in said envelope upstream of said nozzles, outlet means in said envelope downstream of said nozzles and a heating element attached to the bottom of said envelope, wherein said bottom of the envelope is prOvided with a plurality of concentric annular hollow projections which are recessed from the underside and arranged so that said projections are located between said chimneys, and said heating element is provided with a plurality of concentric annular projections which are formed integrally with the body of said element and shaped to fit within the recesses of said hollow projections on the bottom of said envelope, said projections of the heating element being positioned in said recesses and in thermally coupled relationship thereto. 